More environmental sounds would be interesting (birds chirping, etc.) but I don't think they would be realistic because TC:E takes place in mostly deserted areas and most animals and people will run from the sounds of a firefight. My idea is more that footsteps, reloads and other sounds that already exist should be audible from further away than they currently are. It may also be useful to have more footfall sounds than the ones that currently exist, like scraping footsteps over gravel surfaces. IRL it's easy to hear someone running through a gravel driveway several yards away, but in the game it's impossible. Realistic sound would add more strategic depth and make it possible to sneak up on enemies (or allies...) using the sounds they make as a guide.